I say one is a graymuzzle when their beard (or hair, for the women folk) gets gray in it.
But seriously, am I the only one that find the whole "graymuzzle" phrase silly. There is no consensus on when one goes from just a furry to a graymuzzle either - I have seen after 10 years, or at 30, or at 40.
Far as I see it, I don't think at any age does it make a difference. If someone wants to wear a tail and ears, it doesn't matter if they are 15, 25, 40, or 90.
I certainly don't plan to just stop enjoying myself because I have already been around 10 years. Or I turn 30. Or 40. Or whatever number you throw on there. I mean, sure, I have a bit of time before then, granted. But I doubt my opinion will change on it before then.